In a turbofan engine, lighter components generally lead to more efficient performance. If less energy is expended to move internal engine parts, more energy is available for useful work. At the same time, the components themselves must be strong enough to withstand operational forces, and types of failure typical for the operating environment of the engine. Safety considerations and regulations based on the frequency and/or severity of possible failure will often dictate that the engine components also be able to withstand other atypical, yet foreseeable events. Because stronger and lighter components are often more expensive, a balance must be struck between efficiency, safety, and cost.
Few locations in an aircraft are more representative of efforts to optimize the balance between efficiency, safety, and cost than the engine. While lighter materials are preferable to improve efficiency, the high risk of severe consequences from engine damage will require that the engine be made of components having additional margins of safety. Combining parts having both high strength and low density greatly restricts material choices and increases costs. Not infrequently, processing these strong and light materials such as titanium or composites is also complex and expensive.
Being designed to pull vast quantities of air through the bypass section to generate thrust, blades in the fan section of the engine are the first line of defense for the engine and are highly susceptible to both small and large scale damage from objects pulled in with the surrounding air, including bird impact damage.
Small scale blade damage causes performance deterioration and increases the number of potential crack initiation sites, while large scale damage includes blade deformation and failure. Small impacts can also lead to large scale damage by serving as crack initiation sites. Larger impacts, such as ingestion of birds can cause one or more blades to deform or break in a single event. Regulations are in place to limit the frequency and severity of single event failures because of the increased risk of emergency landings and catastrophic failure.
Blades made entirely from high-strength materials, such as titanium or titanium alloys to name just two non-limiting examples, have been proven to offer sufficient hardness to resist erosion and foreign object damage. But titanium alloys are often expensive to purchase and manipulate into a finished blade. And while titanium has a relatively low density compared to a number of metals, the weight of titanium fan blades are significant contributors to overall engine weight. Fiber composites offer significant weight savings relative to titanium and its alloys, but are far more expensive and do not offer the same resiliency.
One technique of reducing the weight of a blade is to use a lower-density metallic material for the airfoil body. As described earlier, composite blades are extremely light, but are far more complex and expensive to produce relative to titanium blades. Small composite blades do not generally achieve sufficient weight savings to merit the additional complexity and cost.
Forming the blade from a lightweight metallic material can reduce cost and weight over a titanium blade. But without additional support or reinforcement, airfoils made solely from most lightweight metals or alloys do not offer sufficient strength and longevity for long-term use.
Multi-material assembled fan blades, consisting of, but not limited to, a sheath and a blade body made of dissimilar conductive materials, such as metals and/or composites, create a galvanic potential. Currently, a non-conductive adhesive is used to bond the sheath to the blade. The non-conductive adhesive therefore provides an insulative layer that prevents the flow of electrons in the potential galvanic current. This adhesive can have gaps in coverage, or a point contact between the sheath and the blade body can occur upon extended field operation, allowing electrons to flow between the two dissimilar materials, which can potentially lead to corrosion.
Various designs for providing a sheath for use on a fan blade have been proposed, but improvements are still needed in the art.